1. Field of the Invention
The present invention relates to magnetoresistive sensors for reading magnetically-recorded information from data storage media, and particularly to CPP (Current-Perpendicular-to-Plane) spin valve read sensors for direct access storage device (DASD) systems.
2. Description of the Prior Art
By way of background, magnetoresistive spin valve sensors have been proposed for use as magnetic field sensors (e.g., read head sensors) in DASD systems, such as disk drives. A spin valve sensor is a variable current device formed by a magnetically pinned ferromagnetic layer and a magnetically free ferromagnetic layer separated by an electrically conductive spacer layer. An antiferromagnetic pinning layer is used to exchange-bias the pinned layer so that its magnetic moment is pinned 90xc2x0 to an air bearing surface (ABS) of the sensor that magnetically interfaces with an opposing magnetic storage medium during drive operation.
The magnetic moment of the free layer is free to rotate in positive and negative directions from a zero bias point position in response to positive and negative magnetic domains recorded on the magnetic storage medium. The zero bias position is the position of the magnetic moment of the free layer when the sensor is in a quiescent state and no external magnetic fields are present. The magnetic moment of the free layer when the sensor is in its quiescent state is preferably perpendicular to the magnetic moment of the pinned layer and parallel to the ABS. This allows for read signal asymmetry upon the occurrence of positive and negative magnetic field incursions of a rotating disk.
Electrical leads and/or shields are positioned to make electrical contact with the ferromagnetic layers. In a CIP (Current-In-Plane) spin valve sensor, the leads are arranged so that electrical current passes through the sensor in a direction that is parallel to the plane of the pinned and free layers. In a CPP sensor, the leads are arranged to induce a sense current that passes perpendicularly through the spacer layer from the pinned ferromagnetic layer to the free layer. In either case, when the sense current passes through the sensor, a readback signal is generated which is a function of the resistance changes that result when the magnetic moment of the free layer rotates relative to the pinned layer magnetic moment under the influence of recorded magnetic domains. Resistance is lower when the relative magnetic moments are parallel and higher when the magnetic moments are antiparallel.
Because of their superior sensitivity and higher magnetoresistance (xcex94R/R), CPP spin valves are generally preferred over CIP spin valves for future products. High magnetoresistance equates to increased sensitivity of the spin valve sensor to magnetic flux incursions from the rotating storage medium. Other parameters that affect performance are the coupling field (He) between the pinned layer and the free layer across the spacer layer, the coercivity (Hc) of the free layer, and the free layer""s magnetostriction (xcex). It is desirable to minimize the coupling field He insofar as this parameter affects the bias point and sensitivity of the sensor. Ideally, the value the coupling field He is negative. It is also preferable that the coercivity parameter Hc be as low as possible. The magnetostriction value xcexis preferably a negative number.
Commonly assigned U.S. Pat. No. 6,317,299 of M. Pinarbasi (one of the co-inventors named herein), discloses a pining layer seed for increasing the magnetoresistance of a CIP spin valve sensor having a PtMn (platinum manganese) antiferromagnetic pinning layer. The preferred seed layer of the ""299 patent comprises a first seed layer of CoFeB (cobalt iron boron), a second seed layer of NiMnO (nickel manganese oxide) and a third seed layer of Al2O3 (aluminum oxide). U.S. Pat. No. 6,201,671 of M. Pinarbasi similarly discloses a pinning layer seed for increasing the magnetoresistance of a CIP spin valve sensor having an antiferromagnetic pinning layer formed from NiO (nickel oxide). The preferred seed layer of the ""671 patent comprises a metal oxide, such as TayOx (tantalum oxide).
While it would be desirable to take advantage of the pinning seed layers of the ""299 and ""671 patents in a CPP sensor, the use of an oxide seed layer in a CPP configuration is precluded because it would block the sense current.
Accordingly, a need exists for a CPP sensor configuration that increases magnetoresistance in similar fashion to the oxide pinning layer of the ""299 and ""671 patents, but without the incompatibility of such oxides relative to perpendicular-to-plane current flow. What is required in particular is a pinning layer that is electrically conductive relative to an applied CPP sensing signal and which provides desired magnetoresistance-enhancing properties.
The foregoing problems are solved and an advance in the art is obtained by a novel CPP geometry spin valve sensor for sensing magnetically recorded information on a data storage medium. The sensor includes an electrically conductive, multilevel seed layer that interfacially engages an antiferromagnetic pinning layer of the sensor and increases the sensor""s magnetoresistance while maintaining acceptable values of 1) the coercivity (Hc) of the ferromagnetic free layer of the sensor, 2) the magnetic coupling field (He) between the sensor""s ferromagnetic pinned layer and the free layer, and 3) the free layer""s magnetostriction (xcex).
The seed layer preferably comprises a first Ru/Si seed layer portion formed by alternating sub-layers of ruthenium (Ru) and silicon (Si), and a second NiFeCr seed layer portion. The Ru/Si seed layer portion can be applied on a bottom lead or shield of the sensor, preferably using ion beam deposition. The Ru/Si seed layer portion may have a total thickness ranging between about 10-50 xc3x85. The Ru and Si sub-layers are applied in alternating fashion, preferably beginning with an Si sub-layer. In exemplary configurations, the Ru/Si seed layer portion comprises three Si layers and two Ru layers that are each about 6-8 xc3x85 thick. The NiFeCr seed layer portion is formed over the Ru/Si seed layer portion such that it interfacially engages the Ru/Si seed layer portion and an overlying PtMn antiferromagnetic pinning layer. The PtMn antiferromagnetic layer itself interfacially engages the sensor""s ferromagnetic pinned layer(s).
The invention further contemplates an integrated read/write head and a disk drive that respectively include a spin valve sensor as summarized above.